High speed laser scanning inspection system

ABSTRACT

An optical inspection system rapidly evaluates a substrate by illumination of an area of a substrate larger than a diffraction-limited spot using a coherent laser beam by breaking temporal or spatial coherence. Picosecond or femtosecond pulses from a modelocked laser source are split into a plurality of spatially separated beamlets that are temporally and/or frequency dispersed, and then focused onto a plurality of spots on the substrate. Adjacent spots, which can overlap by up to about 60-70 percent, are illuminated at different times, or at different frequencies, and do not produce mutually interfering coherence effects. Bright-field and dark-field detection schemes are used in various combinations in different embodiments of the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional patentapplication No. 60/378,400 filed May 6,2002 titled “High speed laserinspection system” and claims the benefit of U.S. provisional patentapplication No. 60/378,721 filed May 7, 2002 titled “Optical techniquefor detecting buried defects in opaque films”.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to systems and methods fordetecting irregularities on a substrate. More particularly, thisinvention relates to systems and methods for detecting irregularities onthe surface of silicon wafers or photomasks.

[0004] 2. Description of the Related Art

[0005] Semiconductor wafers are inspected prior to, during, and afterpatterning procedures. Optical inspection systems typically employillumination optics and collection-detection optics for directingincident light from a light source onto a wafer to be inspected, andobserving returned light.

[0006] Imaging techniques are often employed in modern inspectionsystems. These systems are classified according to the direction of theillumination with respect to the collection optics. If the illuminationimpinges upon the substrate from a direction such that the transmittedor specularly reflected light is collected by the imaging optics, thesystem is termed “bright field” (BF). If, on the other hand, thetransmitted or specularly reflected light arrives from a direction,which is outside the collection angle of the collection optics, thesystem is termed “dark field” (DF).

[0007] Today, semiconductor wafers are inspected using bright-fieldtechniques, dark-field techniques, or combinations thereof. Coherentlight is commonly employed for illumination.

[0008] Lasers, which are commonly used in inspection systems produceundesirable coherent phenomena in imaging systems, such as ringing ofedges and speckles. Schemes exist for destroying the coherence of lasersources, but they inevitably add to the system's complexity and reduceoptical power. Laser-scanning inspection systems typically employ afocused laser spot scanning over the sample. The reflected or scatteredlight is collected by a detector, which may be non-imaging, e.g., aphotomultiplier tube, partially imaging, e.g., a linear CCD, or fullyimaging, e.g., an area CCD. Each of these entails certain advantages andlimitations.

[0009] With a non-imaging detector, the system resolution is determinedsolely by the illuminated area, as all the collected light is integratedinto a single signal. This scheme precludes multi-spot, line and areaillumination schemes. Throughput is limited by the spot size and thescan rate. Usually the beam is scanned over the sample using a rotatingpolygon mirror, acousto-optic device scanner, or oscillatinggalvanometric scanning mirror.

[0010] With a partially imaging or fully imaging detector, light iscollected simultaneously from a larger region of the sample so thatmultiple spots may be illuminated simultaneously, multiplying theprevious throughput by the number of illumination spots. However, ifthese spots are not spatially separated, image distortions may occur dueto coherent interference effects.

[0011] It may be advantageous to simultaneously collect both thereflected or bright-field image, at high spatial resolution at highthroughput, and the scattered or dark-field image, at lower spatialresolution at low throughput. The advantage is related to the differencein throughput achievable with optimal detectors for the bright-fieldimage (partially imaging or fully imaging), as compared with thedark-field (non-imaging) . One method for achieving simultaneousbright-and dark-field detection is addressed in commonly assigned U.S.Pat. No. 6,122,046 to Almogy, the disclosure of which is hereinincorporated by reference, wherein a large spot on a substrate isilluminated using optics having a low numerical aperture (NA).Dark-field detection is achieved by a non-imaging detector. Thedark-field resolution is determined by the illumination spot size. Thebright-field signal is collected with high NA optics, and imaged onto anarea detector, providing improved resolution relative to the dark fielddetector by a factor that is the ratio of the collection NA to theillumination NA. However, when using imagine detectors, the bright-fieldimages may suffer undesirable distortion due to coherence effects in theform of ringing at feature edges, if there is mismatch between the NA ofthe collection optics and the imaging optics. When detection isperformed with non-imaging detectors, which generally have lowerresolution than imaging detectors, these coherence effects can usuallybe disregarded.

SUMMARY OF THE INVENTION

[0012] The invention provides an apparatus and method for rapidlyinspecting a substrate by illuminating an area of a substrate that islarger than a diffraction-limited spot without undesirable interferenceeffects.

[0013] The invention provides an apparatus and method for illuminating alarge area of a substrate using high-NA imaging optics and using imagingdetectors without incurring undesirable coherence interference effects.

[0014] The invention provides an apparatus and method for breakingtemporal or spatial coherence of a pulsed beam of coherent light. Insome embodiments of the present invention, ultrafast scanning isachieved using picosecond or femtosecond pulses from a modelocked lasersource. The pulsed laser beam is split into a plurality of beamlets,each of which follows a path having a uniquely different length. Thebeamlets are respectively focused onto a plurality of spots. Adjacentspots are illuminated at different times, and therefore do not mutuallyinterfere.

[0015] Other embodiments of the invention exploit the large spectralbandwidth produced by laser pulses, which are femtoseconds orpicoseconds in duration. In some of these embodiments, beamlets havingdifferent frequencies are dispersed temporally. In other embodiments,the beamlets having different frequencies are dispersed spatially. Inany case, the dispersion illuminates different spots on the substrate,without interference.

[0016] In the above-noted embodiments, high-resolution detection may beperformed with an imaging detector. Additionally or alternatively, oneor more single-element detectors having a high NA may be employed. Inthose embodiments employing frequency dispersion, different singleelement detectors can be sensitive to different frequencies. Optionally,dark-field detectors can be additionally provided. Typically non-imagingdark-field detectors have a high NA, and operate by simultaneouslycollecting the light scattered from all illuminated spots. The spots areusually orthogonal or somewhat oblique to a primary scan direction. TheNA of an imaging dark-field detector is typically matched to theillumination spot size.

[0017] The invention provides an apparatus for optical inspection,including a coherent light source for producing a pulsed beam, and abeam converter or manipulator for dividing the beam into a plurality ofspatially separated beamlets that are incident on a substrate to beinspected. Each of the beamlets illuminates a different spot on thesubstrate, such that responsively to the beam converter, there issubstantially no mutual interference between the beamlets thatilluminate adjacent spots. The apparatus includes a scanner fordisplacing the beamlets across the substrate, and a detector, which isdisposed in a path of light of the beamlets that is reflected from thesubstrate.

[0018] According to one aspect of the apparatus, the centers of at leasttwo of the different spots are spaced apart by no more than about 3 spotdiameters. Different spots may be in mutual contact, and may overlap byup to about 60-70 percent.

[0019] According to one aspect of the apparatus, the light source is amodelocked laser.

[0020] According to another aspect of the apparatus, the scannerincludes a first scanner for displacing the beamlets in a primaryscanning direction and a second scanner for displacing the beamlets in asecondary scanning direction.

[0021] According to a further aspect of the apparatus, the detector is abright-field detector.

[0022] According to yet another aspect of the apparatus, the detectoralso includes a dark-field detector.

[0023] According to still another aspect of the apparatus, the detectoris a single-element detector.

[0024] According to an additional aspect of the apparatus, the detectorincludes a plurality of single-element detectors.

[0025] According to another aspect of the apparatus, each of thesingle-element detectors is sensitive to a different waveband.

[0026] According to one aspect of the apparatus, the detector is animaging detector.

[0027] According to still another aspect of the apparatus, the detectoris an imaging detector in a first scanning direction and a non-imagingdetector in a second scanning direction.

[0028] According to a further aspect of the apparatus, the beamlets areincident normally to the substrate.

[0029] According to yet another aspect of the apparatus, the beamletsare incident obliquely to the substrate.

[0030] According to one aspect of the apparatus, the beam converterincludes a wavelength beam converter.

[0031] According to another aspect of the apparatus, the beam converterincludes a temporal beam converter.

[0032] According to yet another aspect of the apparatus, the temporalbeam converter includes a plurality of beamsplitters and a plurality ofretroreflectors, wherein a light path of each of the beamlets extendsthrough at least one of the beamsplitters and through one of theretroreflectors.

[0033] According to yet another aspect of the apparatus, the temporalbeam converter includes a plurality of optical fibers has differentlengths, wherein a light path of each of the beamlets extends through acorresponding one of the fibers.

[0034] According to still another aspect of the apparatus, the temporalbeam converter includes a plurality of edge filters and a plurality ofretroreflectors, wherein a light path of each of the beamlets extendsthrough at least one of the edge filters and through one of theretroreflectors.

[0035] The invention provides a method of optical inspection, which iscarried out by emitting a pulsed beam of coherent light, dispersing thebeam into a plurality of spatially separated beamlets, and impinging thebeamlets onto a substrate to be inspected, wherein each of the beamletsilluminates a different spot on the substrate and there is substantiallyno mutual interference between the beamlets that illuminate adjacentspots on the substrate. The method is further carried out by displacingthe beamlets across the substrate, and detecting reflections of thebeamlets from the substrate.

[0036] According to one aspect of the method, the centers of at leasttwo of the different spots are spaced apart by no more than about 3 spotdiameters. Different spots may be in mutual contact, and may overlap byup to about 60-70 percent.

[0037] In a further aspect of the method converting is performed bytemporal dispersion.

[0038] In yet another aspect of the method converting is performed bywavelength dispersion.

[0039] In an additional aspect of the method the beamlets are displacedin a primary scanning direction and in a secondary scanning direction.

[0040] In still another aspect of the method scattered light of thebeamlets from the substrate is detected.

[0041] Yet another aspect of the method detecting is performed using asingle-element detector.

[0042] A further aspect of the method detecting is performed using aplurality of single-element detectors.

[0043] According to another aspect of the method, each of thesingle-element detectors is sensitive to a different waveband.

[0044] In one aspect of the method detecting is performed using animaging detector.

[0045] In an additional aspect of the method detecting is performedusing an imaging detector in a first scanning direction and anon-imaging detector in a second scanning direction.

[0046] According to an additional aspect of the method, the beamlets areimpinged normally to the substrate.

[0047] According to still another aspect of the method, the beamlets areimpinged obliquely to the substrate.

[0048] According to an additional aspect of the method converting isperformed by imposing a frequency chirp on pulses of the beam.

[0049] The invention provides an apparatus for optical inspection,including a coherent light source for producing a pulsed beam, and awavelength beam converter for dividing the beam into a plurality ofspatially separated beamlets that are incident on a substrate to beinspected. Each of the beamlets has a different waveband and illuminatesa different spot on the substrate. The apparatus includes a scanner fordisplacing the beamlets across the substrate, and a detector disposed ina path of reflected light from the substrate of the beamlets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] For a better understanding of these and other objects of thepresent invention, reference is made to the detailed description of theinvention, by way of example, which is to be read in conjunction withthe following drawings, wherein like elements are given like referencenumerals, and wherein:

[0051]FIG. 1 is a high level schematic illustration of an opticalinspection system, which is constructed and operative in accordance witha disclosed embodiment of the invention;

[0052]FIG. 2 is an enlarged schematic view of optics in the system shownin FIG. 1, illustrating illumination of a substrate in accordance with adisclosed embodiment of the invention;

[0053]FIG. 3 is a schematic illustration of an optical inspection systememploying multiple pulsed, temporally dispersed and spatially separatedbeamlets, which is constructed and operative in accordance with adisclosed embodiment of the invention;

[0054]FIG. 4 is a schematic illustration of an optical inspection systememploying multiple pulsed spatially separated beamlets, dispersedtemporally and by waveband, which is constructed and operative inaccordance with an alternate embodiment of the invention;

[0055]FIG. 5 is a schematic illustration of an optical inspectionsystem, which is constructed and operative in accordance with analternate embodiment of the invention;

[0056]FIG. 6 is a schematic illustration of an optical inspection systememploying multiple pulsed beamlets that are spatially dispersedaccording to waveband, which is constructed and operative in accordancewith an alternate embodiment of the invention;

[0057]FIG. 7 is a schematic illustration of an optical inspection systememploying multiple pulsed spatially separated beamlets, dispersedtemporally and by waveband, which is constructed and operative inaccordance with an alternate embodiment of the invention;

[0058]FIG. 8 is a plot of optical activity in the system shown in FIG.7, illustrating temporal dispersion of beamlets;

[0059]FIG. 9 is a schematic illustration of an optical inspection systememploying multiple pulsed spatially separated beamlets, dispersedtemporally and by waveband, which is constructed and operative inaccordance with an alternate embodiment of the invention;

[0060]FIG. 10 is a composite schematic representation of detectorarrangements, which are used in various embodiments of the invention;

[0061]FIG. 11 is a schematic illustration of a detection subsystem foruse in optical inspection systems according to various embodiments ofthe invention; and

[0062]FIG. 12 is a schematic illustration of an alternate detectionsubsystem for use in optical inspection systems for use in opticalinspection systems according to various embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0063] In the following description, numerous specific details are setforth in order to provide a thorough understanding of the presentinvention. It will be apparent to one skilled in the art, however, thatthe present invention may be practiced without these specific details.In other instances well-known circuits, and control logic have not beenshown in detail in order not to unnecessarily obscure the presentinvention.

[0064] Notation.

[0065] The following notation is used herein:

[0066] c is the speed of light in a relevant medium;

[0067] R is the scan rate in pixels/second along a primary scandirection;

[0068] r is the scan rate in pixels/second along a secondary scandirection, transverse to the primary scan direction;

[0069] T is the time between successive pulses emitted from a modelockedlaser; and

[0070] τ is the duration in time of a pulse emitted from the modelockedlaser.

[0071] Overview.

[0072] Turning now to the drawings, reference is initially made to FIG.1, which is a high level schematic illustration of an optical inspectionsystem 10 that is constructed and operative in accordance with adisclosed embodiment of the invention. Using the system 10, ultrafastscanning is achieved using a pulsed beam 12 of coherent light, shownrepresentatively as a pulse 14. The pulse 14 may typically vary induration from a few picoseconds to femtoseconds, and is emitted from amodelocked laser source 16. Alternatively, longer or shorter pulsedurations may be used in some applications.

[0073] Suitable values for the pulse interval T of the beam 12 are 1-20nsec, In the embodiments disclosed below utilizing temporal dispersion,but not spectral dispersion, pulse durations of 10-100 psec are suitablefor the pulse 14. In embodiments that employ spectral separation, pulsedurations τ from 100 fsec-10 psec are suitable.

[0074] For embodiments in which femtosecond pulses are generated, themodel Vitesse 800, available from Coherent Inc., 5100 Patrick HenryDrive, Santa Clara, Calif. 95054 U.S.A., is suitable for the lasersource 16. The model Vitesse 8000 produces pulses having an 8 nmbandwidth, which allows for temporal dispersal at a resolution of 10-20beamlets per pulse interval, as is disclosed in further detailhereinbelow.

[0075] For embodiments in which picosecond pulses are generated, thelaser source 16 can be the model IC-532-1000, manufactured by High-QLaser Production GmbH, Kaiser-FranzJosef-Str. 61, A-6845 Hohenems,Austria. The model IC-532-1000 operates with a pulse interval of 12.5nsec and a pulse duration of 6 psec. Although the system 10 is capableof generating up to 1000 differently timed pulsed beamlets during eachpulse interval of the laser source 16, each beamlet being separated fromthe next by twice the pulse duration, it is practical to operate themodel IC-532-1000 in arrangements producing 10-20 individual beamletsper pulse interval.

[0076] The beam 12 enters a beam converter 18, which divides the beam 12into a plurality of spatially separated beamlets 20, 22, 24. Thebeamlets 20, 22, 24 are displaced relative to a substrate 26 by ascanner 28, and are impinged on the substrate 26 by directing optics 30and focusing optics 32, so that each beamlet illuminates a differentspot. The beamlets 20, 22, 24 should optimally focus todiffraction-limited spots, which overlap by about 60-70% in order togenerate a uniformly illuminated line.

[0077] In some embodiments the beamlets 20, 22, 24 are dispersed by thebeam converter 18 according to wavelength. In other embodiments thebeamlets 20, 22, 24 are dispersed temporally, so that pulses thereineach occupy a unique time interval. In still other embodiments, bothtemporal and wavelength dispersion are produced in combination by thebeam converter 18. Different embodiments of the beam converter 18 aredisclosed in further detail hereinbelow.

[0078] The scanner 28, which can be any suitable optical deflectionsystem, moves the beamlets 20, 22, 24 across the substrate 26 in aprimary scanning direction. Movement in a secondary scanning direction,which is usually orthogonal to the primary scanning direction, istypically achieved by mechanical displacement of the substrate 26relative to the focusing optics 32. This can be accomplished by amechanical stage (not shown) . In some embodiments, the scanner 28 canbe a 2-dimensional scanner, in which case the mechanical stage can beomitted. Light within the specular angular range returns from thesubstrate 26 to beam processing optics 34 and is detected and processedby a bright-field detector 36 and a data sampler 38. The detector 36 canbe an imaging detector, and should be CCD or CMOS-based. The number ofdetector elements is application dependent. A configuration of at least2000×n pixels is typical, where n is the number of beamlets in thesecondary scanning direction. The detector 36 should be capable oftransferring data at the rate (nR) pixels/sec. As used herein, the terms“primary scanning direction” and “secondary scanning direction” are usedarbitrarily to distinguish scanning directions. These terms otherwisehave no physical meanings with respect to the actual configuration ofthe system 10.

[0079] Optionally, a dark-field detector 40 and a data sampler 42 may beprovided for light that is scattered by the substrate 26. The techniquesdisclosed in the above-noted U.S. Pat. No. 6,122,046 are suitable whenboth the detector 36 and the detector 40 are operated together. Thedetector 40 can be a non-imaging detector, such as the model R6355photomultiplier tube, manufactured by Hamamatsu Photonics K. K., 314-5,Shimokanzo, Toyooka-village, Iwata-gun, Shizuoka-ken, 438-0193, Japan.

[0080] An imaging dark-field detector may be used as the detector 40. Itcan have a relatively low resolution, as there is a low signal-to-noiseratio. In such case, the NA of the collection optics should be matchedto the total illumination on the substrate. A low NA is typicallychosen, in order to spatially integrate over the secondary scanningdirection, that is the linear area that is illuminated at any particulartime during a continuous scanning operation.

[0081] Reference is now made to FIG. 2, which is an enlarged schematicview of optics in the system 10 (FIG. 1), illustrating illumination ofthe substrate 26 in further detail. The beamlets 20, 22, 24 illuminatespots 44, 46, 48 on the substrate 26. With reference to the coordinatesystem shown in FIG. 2, scanning motion in the primary direction alongthe X-axis is indicated by a double-headed arrow A, while scanningmotion in the secondary direction along the Z-axis is shown by asingle-headed arrow B. The individual spots 44, 46, 48 should partiallyoverlap, as noted above, in order to produce both a uniform line sectionand to sample the substrate within the Nyquist frequency, as iswell-known from sampling theory. The overlap should be about 60-70%.Below this range, performance progressively degrades. However, in someembodiments, using either temporal or spectral separation, some benefitof the system 10 will be seen, even when the centers of non-contiguousneighboring spots are spaced apart by up to 3 to 4 spot sizes ordiameters.

[0082] In high-speed scanning applications, which are required in aproduction environment, movements in the primary and secondary scanningdirections are coordinated such that there is an overlap of the about60-70% between the spot illuminated by the last beamlet of one secondaryscan, and the first beamlet of the next. This can be appreciated in FIG.2, in which the last illuminated spot on a strip 50, that is the spot48, overlaps a first spot 52 on an adjacent strip 54.

[0083] First Embodiment

[0084] Reference is now made to FIG. 3, which is a schematicillustration of an optical inspection system 56, which is constructedand operative in accordance with a disclosed embodiment of theinvention.

[0085] The beam 12 is split into a plurality of beamlets 58, 60, 62 bybeamsplitters 64, 66, 68, respectively. The relative reflectance andtransmittance of each of the beam-splitters 64, 66, 68 are typically(but not necessarily) chosen so that all the beamlets 58, 60, 62 haveequal intensities. The beamlets 58, 60, 62 are received respectively byretro-reflectors 70, 72, 74. The beamlets 58, 60, 62 are then directedto a reflector 76 by reflectors 78, 80, 82, after which they passthrough the scanner 28, beam processing optics 34 and focusing optics32, and impinge on the substrate 26. The retroreflectors 70, 72, 74 formfree-space delay lines. They are disposed so that the optical paths ofthe beamlets 58, 60, 62 are of different lengths. Thus, thebeamsplitters 64, 66, 68 and the retroreflectors 70, 72, 74 cooperate toconstitute a temporal manipulator for the beam 12.

[0086] The beamlets 58, 60, 62 are focused onto adjacent spots on thesubstrate 26, but since they arrive at different times, they do notmutually interfere. In some embodiments, the beamlets 58, 60, 62 areoblique to the substrate 26. In other embodiments the beamlets 58, 60,62 are normal to the substrate 26. While it is possible to produce aseries of beamlets, e.g., 20, each having a different time delay orwavelength, in practice it may be more convenient to generate only a fewat a time, e.g., five, each having a different delay and/or waveband,the spots typically, but not necessarily, overlapping by about 60-70% toproduce a short linear segment in which the beams have no mutualinterference, This segment can be replicated 4 times in differentlocations to produce a line of illumination, on the substrate 26equivalent to 20 beamlets. Beamlets in different segments having thesame delays or wavebands are sufficiently spaced apart from one anotherto avoid mutual interference.

[0087] The path lengths of the beamlets 58, 60, 62 are set to differ bya factor of at least 2τ/c. For a 10 psec pulse, the path differencebetween adjacent spots is approximately 6 mm in vacuum, andapproximately 4 mm in glass (refractive index=1.5) . The total number ofspots n_(max) in the secondary scanning direction is limited ton_(max)=T/2τ, where T is the time between successive pulses exiting thelaser. The effective cross-scan rate for the actual number of spots n isr=n/T, and the effective total scan rate R_(eff) increases toR_(eff)=Rn, as compared with the total scan rate R that is achieved whenonly one spot is illuminated.

[0088] Second Embodiment

[0089] Reference is now made to FIG. 4, which is a schematicillustration of an optical inspection system 84, which is constructedand operative in accordance with an alternate embodiment of theinvention. The description of FIG. 4 should be read in conjunction withFIG. 3. The arrangement of the system 84 is similar to the system 56,except now, the beamsplitters 64, 66, 68 (FIG. 3) are replaced by aseries of reflective edge filters 86, 88, 90, which produce beamlets 92,94, 96, each having a unique waveband.

[0090] The ultrafast pulse 14 inherently contains a large spectralbandwidth, with a minimum bandwidth Δv given by: Δv·τ≅1. The beamlets92, 94, 96 are spatially separated, and not only are dispersedtemporally as disclosed in the discussion of the embodiment of FIG. 3,but are also distributed according to wavelength, using the edge filters86, 88, 90, which disperse the beamlets 92, 94, 96 according towavelength. For example, a typical 100 fsec pulse with a centralwavelength of 800 nm has an operational spectral width of about 20 nm.In the system 84, a bright-field detector unit 98 may include aplurality of individual detectors, each sensitive to a particularwaveband, or it may include one or more detectors with widebandsensitivity.

[0091] Dispersing the beam 12 into different wavelength components issufficient to break the coherence between n adjacent spots, as the spotsare created by light having different frequencies, which inherentlycannot interfere. Even in embodiments of the system 84 in which thedelay paths of the beamlets 92, 94, 96 are equalized, and the ndifferent spots are not actually separated in time, the effective scanrate is still R_(eff)=Rn. An advantage of simultaneous temporal andfrequency dispersion is the ability to replicate a frequency-dispersedbeamlet set at different temporal delays in order to increase the lengthof a scan line, and thereby increase scanning throughput.

[0092] In applications in which an imaging detector is used as thedetector unit 98, any needed compensation for different illuminationintensities of the returning beamlets 92, 94, 96 can be achieved byincorporating attenuators (not shown) in the detector unit 98.

[0093] Third Embodiment

[0094] Reference is now made to FIG. 5, which is a schematicillustration of an optical inspection system 100, which is constructedand operative in accordance with an alternate embodiment of theinvention. The system 100 is similar to the system 84 (FIG. 4). However,different time delays for the beamlets 92, 94, 96 are now achieved bytransmitting the beamlets 92, 94, 96 respectively through optical fibers102, 104, 106, each fiber having a different length. Thus, the opticalpaths followed by the beamlets 92, 94, 96 have unique lengths.

[0095] Fourth Embodiment

[0096] Reference is now made to FIG. 6, which is a schematicillustration of an optical inspection system 108, which is constructedand operative in accordance with an alternate embodiment of theinvention. The description of FIG. 6 should be read in conjunction withFIG. 3.

[0097] The beam 12 passes through a prism 110, where it is dispersedspatially according to frequency to form beamlets 112, 114, 116, eachhaving a unique waveband. The prism 110 thus acts as a wavelengthmanipulator for the beam 12.

[0098] The beamlets 112, 114, 116 are scanned by the scanner 28 and areredirected by a reflector 118 through the focusing optics 32 to impingeobliquely or normally on the surface of the substrate 26. Returned lightcan be detected in the same manner as disclosed in the discussion of theembodiment of FIG. 4, detection components being omitted in FIG. 6 forclarity. The embodiment of FIG. 6 has the advantage of structuralsimplicity, but is unable to achieve temporal separation of individualillumination spots. It should also be noted that while discrete beamletsare shown in FIG. 6 for clarity, the prism 110 actually creates acontinuum of illuminated spots, each of which corresponds to the fullnumerical aperture of the transmission optics.

[0099] Fifth Embodiment

[0100] Reference is now made to FIG. 7, which is a schematicillustration of an optical inspection system 120, which is constructedand operative in accordance with an alternate embodiment of theinvention. The description of FIG. 7 should be read in conjunction withFIG. 6.

[0101] The system 120 is similar to the system 108 (FIG. 6), except thata parallel grating pair 122 is now introduced into the path of the beam12. The grating pair 122 stretches the pulse 14, and is configured bychoosing the distance between the pair, the incidence angle of the beam12, and the grating period in a known manner, so as to spread the pulseacross the entire period between successive pulses emitted by the lasersource 16 to produce a frequency-chirped beam 124. The beam 124 transitsthe prism 110, which separates it into angularly scanned beamlets 126,128, 130, each having a unique waveband and a unique time delay. In thisembodiment, the grating pair 122 and the prism 110 together constitute abeam converter or manipulator, manipulating the beamlets 126, 128, 130both by wavelength and by time delay.

[0102] Reference is now made to FIG. 8, which is a composite plotillustrating temporal dispersion of the beamlets 126, 128, 130 (FIG. 7).Peaks 132, 134, 136 correspond to the beamlets 126, 128, 130,respectively. Each peak has a different frequency, and is delayeddifferently from the others. In the system 120, the prism 110 produces afrequency chirped spectral continuum, in which each point of thespectrum has a unique time delay. Discretized representations of thebeamlets 126, 128, 130 (FIG. 7), and the peaks 132, 134, 136 areprovided for clarity of illustration.

[0103] Detection arrangements for the system 120 are disclosedhereinbelow. The spatio-temporal dispersion produced in the system 120is particularly suited to the use of a single fast non-imaging detector,which can follow the temporal scan. It is also possible to use animaging array sensor, with appropriate compensation for the spectralintensity distribution.

[0104] Referring again to FIG. 7, in the system 120 the scan rate in thedirection orthogonal to the primary scan direction can be extremelyhigh, with the total effective scan rate limited only by the modelockedpulse repetition rate. In practice, the pulse repetition rate can be ashigh as 1 GHz, enabling a scan rate of tens or hundreds of gigapixelsper second, depending on the spectral bandwidth. The effectivecross-scan rate for n spots is R=n/T, provided that the chirp issufficient to spread the pulse 14 across the entire period betweensuccessive pulses of the laser source 16. The total effective scan rateis R_(eff)=Rn.

[0105] Sixth Embodiment

[0106] Reference is now made to FIG. 9, which is a schematicillustration of an optical inspection system 138, which is constructedand operative in accordance with an alternate embodiment of theinvention. The description of FIG. 9 should be read in conjunction withFIG. 7.

[0107] The system 138 is similar to the system 120 (FIG. 7), except thatthe grating pair 122 is replaced by a prism pair 140. The prism pair 140produces a group delay dispersion of the beam 12 to form the beam 124.As in the embodiment of FIG. 7, the pulses of the beam 124 are broadenedwhen compared to the pulse 14, and they display a frequency sweep. Therefractive index for the dispersive glass in the prism pair 140 affectsthe chirp, and is chosen according to the spectrum of the pulse 14, andthe time between successive pulses. In this embodiment, the prism pair140 and the prism 110 together constitute a beam converter, manipulatingthe beamlets 126, 128, 130 both by wavelength and by time.

[0108] Detection

[0109] Reference is now made to FIG. 10, which is a composite schematicrepresentation of detector arrangements that are suitable for use in theembodiments disclosed hereinabove. A strip 142 of the substrate isscanned in a primary scanning direction indicated by an arrow C. Thescan comprises a plurality of linear areas of illumination, eachconsisting of a plurality of overlapping spots as disclosed hereinabove.The linear areas are indicated representatively by areas 144, 146.High-resolution bright-field detection, in which the signal-to-noiseratio is very high, may be performed with a known imaging detector 148,e.g., a CCD camera, which collects the light with a long integrationtime. Each spot is collected to a different detector element or group ofdetector elements 150 in the imaging detector. Thus, as the scanproceeds, light returning from the area 144 is collected on the detectorelements 150 located in a region 152 of the detector 148. Then lightreturning from the area 146 is collected on the detector elements 150located in a region 154.

[0110] Alternatively, bright-field detection can be accomplished using alinear detector array 156 having simultaneous parallel readout ofdetector elements 158, arranged along the secondary scanning direction.As the scan proceeds, light returning from the area 144 is collected onthe detector elements 158, which are then read out simultaneously. Next,while the scanner is positioned over the area 146, returning light isagain collected on the detector elements 158, and readout is repeated.The process continues until the strip 142 has been fully scanned. Usingthis arrangement, the collection optics and detector are thus imaging inthe secondary scanning direction, but non-imaging in the primary scandirection. Each of the spots illuminated by the individual beamlets,e.g., beamlets 92, 94, 96 (FIG. 4), is imaged to a separate one of thedetector elements 158 or to an overlapping group of the detectorelements 158, while the collected beam remains essentially stationary onthe linear detector array 156. This can be achieved using cylindricallenses in the collection optics. Unlike a CCD device, which uses aserial readout, the linear detector array 156 should provide parallelreadout of all the elements in order to maintain synchronization withthe scan. An example of such a linear detector array is the Hamamatsu

that the collection NA equal the illumination NA in order to avoidundesirable coherence effects.

[0111] Alternatively, in embodiments employing temporal dispersion, suchas the embodiment of FIG. 3, bright-field detection of a series ofadjacent spots may be performed using a very fast single-elementnon-imaging detector. This is practical, since each spot on thesubstrate is illuminated in its own time slot. The relationship betweenthe NA of the non-imaging detector and the NA of the illuminator isapplication dependent. It is acceptable, but not essential for the NA ofthe collection optics to exceed the NA of the illumination optics. Insome cases, it is actually advantageous for the NA of the collectionoptics to be smaller than the NA of the illumination optics.

[0112] Alternatively, in the disclosed embodiments employing spatialwavelength dispersion, such as the embodiment of FIG. 6, bright-fielddetection of a series of adjacent spots may be performed using aplurality of detectors, each of which is sensitive to a differentwaveband. The discussion of the relationship between the NA of theillumination optics and the NA of the collection optics for imaging andnon-imaging detectors in embodiments employing temporal dispersion isapplicable to embodiments employing wavelength dispersion.

[0113] In some embodiments of the detection subsystems disclosedhereinbelow, high-NA bright field detectors are cooperative withsimultaneously operating low-resolution dark-field detectors. However,it will be appreciated that these embodiments can be readily modified bythe application of ordinary skill in the art to provide high-resolutiondark field detection capability by replicating the bright field detectorapparatus, appropriately disposing the collection optics so as tocollect scattered light.

[0114] Reference is now made to FIG. 11, which is a schematicillustration of a detection subsystem 160 for an optical inspectionsystem employing spatial wavelength dispersion, which is constructed andoperative in accordance with an alternate embodiment of the invention. Aplurality of pulsed beamlets 162, 164, 166, each having a differentwaveband, are directed by focusing optics 32 onto adjacent spots of thesubstrate 26. The beamlets 162, 164, 166 can be normal to the substrate26, or can be obliquely incident thereon. The beamlets 162, 164, 166 maybe created by any of the embodiments disclosed hereinabove that employspatial dispersion according to wavelength, the details of theilluminator being omitted in FIG. 11 for clarity.

[0115] Reflected light corresponding to the beamlets 162, 164, 166returns to the reflector 118. In FIG. 11, the focusing optics 32function as collection optics for the returned light. Alternatively, adifferent system of collection optics (not shown) may be provided. Thebeamlets 162, 164, 166 continue through a series of optical elements168, 170, 172, which sort the beamlets 162, 164, 166 in order of theirrespective wavebands. The elements 168, 170, 172 can be reflective edgefilters. Returning light from each of the beamlets 162, 164, 166, nowisolated by waveband, is received by a respective member of a set ofdetectors 174, 176, 178, each of which is sensitive to the waveband ofits associated beamlet. The detectors 174, 176, 178 can operate at aslower rate than the single-element detector disclosed above, which isused when time dispersion of beamlets is employed.

[0116] Reference is now made to FIG. 12, which is a schematicillustration of a detection subsystem 180 for an optical inspectionsystem employing spatial wavelength dispersion, which is constructed andoperative in accordance with an alternate embodiment of the invention.The subsystem 180 is similar to the subsystem 160 (FIG. 11), except thatwavelength separation of the beamlets 162, 164, 166 is performed by aprism 182. One or more diffraction gratings may be substituted for theprism 182.

[0117] In all the detection schemes disclosed herein, low-NA dark fielddetection may be additionally performed by simultaneously collecting thelight scattered from the multiple illuminated spots.

[0118] It will be appreciated by persons skilled in the art that thepresent invention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

1. An apparatus for optical inspection, comprising a beam manipulatorfor converting a pulsed beam into a plurality of spatially separatedbeamlets that are incident on a substrate to be inspected, wherein atleast some of said beamlets are directed towards different spots on saidsubstrate, wherein centers of at least two of said different spots arespaced apart by no more than about 3 spot diameters, and wherein thereis substantially no mutual interference between said beamlets.
 2. Theapparatus according to claim 1, wherein said at least two of saiddifferent spots are in mutual contact.
 3. The apparatus according toclaim 1, wherein said at least two of said different spots overlap oneanother.
 4. The apparatus according to claim 3, wherein an overlap ofsaid at least two of said different spots is between about 60 percentand 70 percent.
 5. The apparatus according to claim 1, wherein there issubstantially no mutual interference between said beamlets thatilluminate adjacent instances of said different spots on said substrate.6. The apparatus according to claim 1, wherein said beamlets areincident normally to said substrate.
 7. The apparatus according to claim1, wherein said pulsed beam is coherent light.
 8. The apparatusaccording to claim 1, wherein said beamlets are incident obliquely tosaid substrate.
 9. The apparatus according to claim 1, wherein said beammanipulator comprises a wavelength beam converter.
 10. The apparatusaccording to claim 1, wherein said beam manipulator comprises a temporalbeam converter.
 11. The apparatus according to claim 10, wherein saidtemporal beam converter comprises a plurality of beamsplitters and aplurality of retroreflectors, wherein a light path of each of saidbeamlets extends through at least one of said beamsplitters and throughone of said retroreflectors.
 12. The apparatus according to claim 10,wherein said temporal beam converter comprises a plurality of opticalfibers having different lengths, wherein a light path of each of saidbeamlets extends through a corresponding one of said fibers.
 13. Theapparatus according to claim 10, wherein said temporal beam convertercomprises a plurality of edge filters and a plurality ofretroreflectors, wherein a light path of each of said beamlets extendsthrough at least one of said edge filters and through one of saidretroreflectors.
 14. The apparatus according to claim 1, wherein saidbeam manipulator imposes wavelength dispersion and temporal dispersionon said beamlets.
 15. An apparatus for optical inspection, comprising: acoherent light source producing a pulsed beam; a beam manipulator forconverting said pulsed beam into a plurality of spatially separatedbeamlets that are incident on a substrate to be inspected, wherein atleast some of said beamlets are directed towards different spots on saidsubstrate, wherein there is substantially no mutual interference betweensaid beamlets; a scanner for displacing said beamlets across saidsubstrate; and at least one detector that is positioned such as todetect at least two of said beamlets that are reflected from saidsubstrate.
 16. The apparatus according to claim 15, wherein at least twoof said different spots are in mutual contact.
 17. The apparatusaccording to claim 15, wherein at least two of said different spotsoverlap one another.
 18. The apparatus according to claim 17, wherein anoverlap of said at least two of said different spots is between about 60percent and 70 percent.
 19. The apparatus according to claim 15, whereinthere is substantially no mutual interference between said beamlets thatilluminate adjacent instances of said different spots on said substrate.20. The apparatus according to claim 15, wherein said light source is amodelocked laser.
 21. The apparatus according to claim 15, wherein saidscanner comprises a first scanner for displacing said beamlets in aprimary scanning direction and a second scanner for displacing saidbeamlets in a secondary scanning direction.
 22. The apparatus accordingto claim 15, wherein said detector is a bright-field detector.
 23. Theapparatus according to claim 22, wherein said detector further comprisesa dark-field detector.
 24. The apparatus according to claim 22, whereinsaid detector is a single-element detector.
 25. The apparatus accordingto claim 22, wherein said detector comprises a plurality ofsingle-element detectors.
 26. The apparatus according to claim 25,wherein each of said single-element detectors is sensitive to adifferent waveband.
 27. The apparatus according to claim 22, whereinsaid detector is an imaging detector.
 28. The apparatus according toclaim 22, wherein said detector is an imaging detector in a firstscanning direction and a non-imaging detector in a second scanningdirection.
 29. The apparatus according to claim 15, wherein saidbeamlets are incident normally to said substrate.
 30. The apparatusaccording to claim 15, wherein said beamlets are incident obliquely tosaid substrate.
 31. The apparatus according to claim 15, wherein saidbeam manipulator comprises a wavelength beam converter.
 32. Theapparatus according to claim 15, wherein said beam manipulator comprisesa temporal beam converter.
 33. The apparatus according to claim 32,wherein said temporal beam converter comprises a plurality ofbeamsplitters and a plurality of retroreflectors, wherein a light pathof each of said beamlets extends through at least one of saidbeamsplitters and through one of said retroreflectors.
 34. The apparatusaccording to claim 32, wherein said temporal beam converter comprises aplurality of optical fibers having different lengths, wherein a lightpath of each of said beamlets extends through a corresponding one ofsaid fibers.
 35. The apparatus according to claim 32, wherein saidtemporal beam converter comprises a plurality of edge filters and aplurality of retroreflectors, wherein a light path of each of saidbeamlets extends through at least one of said edge filters and throughone of said retroreflectors.
 36. A method of optical inspection,comprising: emitting a pulsed beam of coherent light; converting saidbeam into a plurality of spatially separated beamlets; impinging saidbeamlets onto a substrate to be inspected, wherein each of said beamletsresponsively to said step of converting illuminates a different spot onsaid substrate, wherein centers of at least two instances of saiddifferent spot are spaced apart by no more than about 3 spot diameters,and there is substantially no mutual interference between said beamletsthat illuminate adjacent instances of said different spot on saidsubstrate; displacing said beamlets across said substrate; and detectingreflected light of said beamlets that is reflected from said substrate.37. The method according to claim 36, wherein said at least twoinstances of said different spot are in mutual contact.
 38. The methodaccording to claim 36, wherein said at least two instances of saiddifferent spot overlap one another.
 39. The method according to claim38, wherein an overlap of said at least two instances of said differentspot is between about 60 percent and 70 percent.
 40. The methodaccording to claim 36, wherein said step of converting is performed bytemporal dispersion.
 41. The method according to claim 36, wherein saidstep of converting is performed by wavelength dispersion.
 42. The methodaccording to claim 36, wherein said beam is emitted by a modelockedlaser.
 43. The method according to claim 36, wherein said step ofdisplacing said beamlets is performed by displacing said beamlets in aprimary scanning direction and displacing said beamlets in a secondaryscanning direction.
 44. The method according to claim 36, wherein saidstep of detecting is further performed by detecting scattered light ofsaid beamlets from said substrate.
 45. The method according to claim 36,wherein said step of detecting is performed using a single-elementdetector.
 46. The method according to claim 36, wherein said step ofdetecting is performed using a plurality of single-element detectors.47. The method according to claim 46, wherein each of saidsingle-element detectors is sensitive to a different waveband.
 48. Themethod according to claim 36, wherein said step of detecting isperformed using an imaging detector.
 49. The method according to claim36, wherein said step of detecting is performed using an imagingdetector in a first scanning direction and a non-imaging detector in asecond scanning direction.
 50. The method according to claim 36, whereinsaid beamlets are impinged normally to said substrate.
 51. The methodaccording to claim 36, wherein said beamlets are impinged obliquely tosaid substrate.
 52. The method according to claim 36, wherein said stepof converting is performed by imposing a frequency chirp on pulses ofsaid beam.
 53. An apparatus for optical inspection, comprising: acoherent light source producing a pulsed beam; a wavelength beamconverter for dividing said beam into a plurality of spatially separatedbeamlets that are incident on a substrate to be inspected, each of saidbeamlets having a different waveband and illuminating a different spoton said substrate, wherein centers of at least two instances of saiddifferent spot are spaced apart by no more than about 3 spot diameters;a scanner for displacing said beamlets across said substrate; and adetector disposed in a path of reflected light from said substrate ofsaid beamlets.
 54. The apparatus according to claim 53, wherein said atleast two instances of said different spot are in mutual contact. 55.The apparatus according to claim 53, wherein said at least two instancesof said different spot overlap one another.
 56. The apparatus accordingto claim 55, wherein an overlap of said at least two instances of saiddifferent spot is between about 60 percent and 70 percent.
 57. Theapparatus according to claim 53, wherein said light source is amodelocked laser.
 58. The apparatus according to claim 53, wherein saidscanner comprises a first scanner for displacing said beamlets in aprimary scanning direction and a second scanner for displacing saidbeamlets in a secondary scanning direction.
 59. The apparatus accordingto claim 53, wherein said detector is a bright-field detector.
 60. Theapparatus according to claim 59, wherein said detector further comprisesa dark-field detector.
 61. The apparatus according to claim 59, whereinsaid detector is a single-element detector.
 62. The apparatus accordingto claim 59, wherein said detector is a plurality of single-elementdetectors.
 63. The apparatus according to claim 62, wherein each of saidsingle-element detectors is sensitive said different waveband of acorresponding one of said beamlets.
 64. The apparatus according to claim59, wherein said detector is an imaging detector.
 65. The apparatusaccording to claim 59, wherein said detector is an imaging detector in afirst scanning direction and a non-imaging detector in a second scanningdirection.
 66. The apparatus according to claim 53, wherein saidbeamlets are incident normally to said substrate.
 67. The apparatusaccording to claim 53, wherein said beamlets are incident obliquely tosaid substrate.
 68. The apparatus according to claim 53, wherein saidwavelength beam converter comprises a plurality of reflective edgefilters.
 69. The apparatus according to claim 68, wherein saidwavelength beam converter further comprises a plurality ofretroreflectors, wherein a light path of each of said beamlets extendsthrough at least one of said edge filters and through one of saidretroreflectors.
 70. The apparatus according to claim 53, wherein saidwavelength beam converter comprises a prism.
 71. The apparatus accordingto claim 70, wherein said wavelength beam converter further comprises aparallel grating pair for imposing a frequency chirp on said beam. 72.The apparatus according to claim 70, wherein said wavelength beamconverter further comprises a prism pair for imposing a frequency chirpon said beam.
 73. An apparatus for optical inspection, comprising: acoherent light source producing a pulsed beam; means for dividing saidbeam into a plurality of spatially separated beamlets that are incidenton a substrate to be inspected, each of said beamlets having a differentwaveband and illuminating a different spot on said substrate, whereincenters of at least two instances of said different spot are spacedapart by no more than about 3 spot diameters; a scanner for displacingsaid beamlets across said substrate; and means for detecting reflectedlight of said beamlets that returns from said substrate.
 74. Theapparatus according to claim 73, wherein said at least two instances ofsaid different spot are in mutual contact.
 75. The apparatus accordingto claim 73, wherein said at least two instances of said different spotoverlap one another.
 76. The apparatus according to claim 75, wherein anoverlap of said at least two instances of said different spot is betweenabout 60 percent and 70 percent.
 77. The apparatus according to claim73, wherein said light source is a modelocked laser.
 78. The apparatusaccording to claim 73, wherein said scanner comprises a first scannerfor displacing said beamlets in a primary scanning direction and asecond scanner for displacing said beamlets in a secondary scanningdirection.
 79. The apparatus according to claim 73, wherein saidbeamlets are incident normally to said substrate.
 80. The apparatusaccording to claim 73, wherein said beamlets are incident obliquely tosaid substrate.